The present invention relates generally to the formation of articles with powdered materials and more particularly to an article formed with a powdered material to include a hollow cavity formed therein and a method for forming the same.
Turbine disks and blades are commonly subject to high cycle fatigue failures due to high alternating stresses as a result of resonant vibration and/or fluid-structural coupled instabilities. Turbine disks are typically designed to avoid standing wave diametrical mode critical speeds within the operating speed range. High dynamic response occurs when the backward traveling diametrical mode frequency is equal to the forward speed diameteral frequency which results in a standing wave form with respect to a stationary asymmetric force field. Turbine blades are designed to avoid having any of the blade natural frequencies from being excited by the stationary nozzle forcing frequencies in the operating speed range.
In conventional turbine wheel assemblies, conventional blade dampening techniques are typically employed to reduce the fluid-structure instabilities that results from the aerodynamic forces and structural deflections. Accordingly, it is common practice to control both blade and disk vibration in the gas turbine and rocket engine industry by placing dampers between the platforms or shrouds of individual dovetail or fir tree anchored blades. Such blade dampers are designed to control vibration through a non-linear friction force during relative motion of adjacent blades due to tangential, axial or torsional vibration modes. Blade dampers, in addition to the blade attachments, also provide friction dampening during vibration in disc diametral modes.
Integrally bladed turbine disks (blisks) are becoming increasingly common in the propellant turbopumps of liquid fueled rocket engines and gas turbines. While the elimination of separate turbine blades reduces fabrication costs, the monolithic construction of integrally bladed turbine disks eliminates the beneficial vibration damping inherent in the separately bladed disk construction. Accordingly, the above-mentioned damping mechanism is not heretofore been feasible for integrally bladed turbine disks unless radial slots were machined into the disk between each blade to introduce flexibility to the blade shank. The added complexity of the slots would increase the rim load on the turbine blade and defeat some of the cost, speed and weight benefits of the blisk. Consequently, the lack of a blade attachment interface had resulted in a significant reduction in damping and can result in fluid-structure instabilities at speeds much lower than the disk critical speed and at minor blade resonances.
Other dampening mechanisms have been proposed that typically require multiple machining operations followed by the use of external fastener attachments. These machining operations tend to be rather expensive, thereby negating many of the cost advantages of the integrally-bladed turbine disk. Furthermore, there is a general desire to reduce or eliminate the use of any fasteners which, if over stressed, could possibly break loose and cause damage. Accordingly, there remains a need in the art for an improved vibration dampening mechanism that is cost-effectively integrated into an integrally-bladed turbine disk such that the dampening mechanism is housed within a cavity formed into the integrally-bladed turbine disk.
In one preferred form, the present invention provides a method for forming a hollow cavity in an article. The method includes the steps of providing a preformed article; positioning a hollow structure having an open end and an inside wall at a predetermined position relative to the preformed article; filling a space around at least a portion of the hollow structure with a powdered material, the space abutting the preformed article; and exposing the hollow structure and the powdered material to a pressurized fluid such that the pressurized fluid compacts the powdered material and simultaneously exerts a resisting force onto the inside wall of the hollow structure.
In another preferred form, the present invention provides an article having a first article portion, a second article portion and a hollow structure. The hollow structure has an endless body portion with an inside wall and a stem portion that intersects the body portion and has an open end. The body portion is positioned around a portion of first article portion. The second article portion is formed from a powdered material. The second article portion abuts the first article portion and surrounds the body portion of the hollow structure. The second article portion is consolidated and diffusion bonded to the first article portion in a hot isostatic pressing operation.